1. Field of the Invention
The present invention relates to a magnetic powder composition and, more particularly, to a magnetic powder composition suitable for manufacturing a compressed powder core in which electric insulation between magnetic powder particles is improved.
2. Description of the Prior Art
In the prior art, in electrical instruments such as an electric power converting device, including a device for converting an alternate current to a direct current, a device for converting an alternate current having a certain frequency to another alternate current having a different frequency and a device for converting a direct current to an alternate current such as so called inverter, or a non-contact breaker, etc., there have been employed, as electrical circuit constituent elements thereof, semiconductor switching elements, typically thyristor and transistor, and reactors for relaxation of turn-on stress in a semiconductor switching element, reactors for forced commutation, reactors for energy accumulation or transformers for matching connected to these elements.
Iron cores used in such reactors or transformers are conventionally classified as follows:
(a) So called laminated iron cores produced by laminating thin electromagnetic steel plates or permalloy sheets with an insulating interlayer interposed therebetween.
(b) So called dust cores obtained produced by a powder such as a carbonyl iron or permalloy powder with kaolin or a polymeric binder such as a phenol resin.
(c) So called ferrite cores produced by sintering an oxide magnetic material.
Such iron cores used in reactors or transformers which are connected to the semiconductor switching elements must satisfy specific magnetic property requirements. For example, such an iron core must have good frequency characteristics of magnetic permeability, high magnetic flux density, and small iron loss at high frequencies. Especially when a semiconductor switching element is operated, in addition to a current having a period of a switching frequency, a current having a frequency component which is far higher than the switching frequency, e.g., several tens of kilohertz to 500 kHz or higher, may flow in the iron core. In view of these, the iron core must definitely have good characteristics in a high-frequency range.
Of the three types of iron cores, although laminated iron cores exhibit excellent electrical characteristics within a commercial frequency range, they are subject to a large iron loss within a high-frequency range. In particular, in a laminated iron core, the eddy current loss increases in proportion to a square of the frequency. Furthermore, with an increase in the depth from the surface of the plate or sheet material constituting the iron core, the magnetizing force is less subject to changes due to the skin effect of the iron core material. Therefore, the laminated iron core can only be used at a magnetic flux density which is far lower than a saturated magnetic flux density of the laminated iron core material in a high-frequency range. The laminated iron core also has a very large eddy current loss.
In addition to the above disadvantages, laminated iron cores have a very low effective magnetic permeability at high frequencies as compared to an effective magnetic permeability within a commercial frequency range.
When a laminated iron core having these problems is used for a reactor or transformer connected to a semiconductor switching element through which a high-frequency current flows, the iron core itself must be rendered large so as to compensate for the low effective magnetic permeability and magnetic flux density. When the iron core is thus rendered large, the iron loss of the iron core is increased, and the length of the coil windings wound around the iron core is also increased, thereby increasing copper loss.
Dust cores, as the second types of iron core described above, are also conventionally used as iron cores. For example, Japanese Patent Registration No. 112,235 discloses the manufacture of a dust core for use as an iron core by compressing and forming a mixture of an iron powder or an ion alloy powder with an organic or inorganic binder and heating the formed mixture.
However, a dust core prepared in this manner generally has a low magnetic flux density and a low magnetic permeability. Even a dust core prepared using a carbonyl iron powder having a relatively high magnetic flux density has a magnetic flux density at a magnetizing force of 10,000 A/m of slightly higher than 0.1 T and a magnetic permeability of about 1.25.times.10.sup.-5 H/m. Therefore, in a reactor or transformer using such a dust core as an iron core material, the iron core must be rendered large in order to compensate for a low magnetic flux density and a low magnetic permeability. With such an increase in the size of the iron core, the coil windings become longer, also resulting in a large copper loss of the reactor or transformer.
Ferrite cores, as the third type of iron cores, are frequently used for small electric equipment and have a high specific resistance and relatively good high-frequency characteristics. However, a ferrite core has a low magnetic flux density of about 0.4 T at a magnetizing force of 10,000 A/m. In addition to this, the permeability and the magnetic density at the same magnetizing force change by several tens of percentages within a temperature range of -40.degree. to +120.degree. C., which is the temperature range wherein the iron core is used. Thus, when a ferrite core is used as an iron core material of a reactor or transformer connected to a semiconductor switching element, the iron core must be rendered large to compensate for a low magnetic flux density, resulting in the same problem as with the two other types of iron cores.
Furthermore, since ferrite is a sintered body, the manufacture of a large iron core with ferrite is difficult. For this reason, it is difficult to use a ferrite core for handling high power. Due to the longer coil windings and larger copper loss owing to a low magnetic flux density and the great temperature dependencies of magnetic permeability and magnetic flux density, when a ferrite core is used for a reactor or transformer, the core is subject to great variations in its characteristics. When a ferrite core is compared with an electromagnetic steel plate or the like, it has a higher magnetostriction and generates a higher noise from the iron core.